The present invention relates to the use of microwaves to provide a heating source for chemical reactions. The technique is generally referred as "microwave assisted chemistry" and has found wide application in various chemical reactions such as digestion, extraction, drying, moisture analysis, Kjeldahl reactions, sample preparation for further analysis such as spectroscopy, and other techniques.
The nature of microwave radiation provides a number of advantages in conducting chemical reactions. First, in contrast to some conventional heating techniques in which a vessel is heated externally in order to in turn heat the reagents inside, microwave radiation heats the reagents directly and can be carried out--and indeed is desirably carried out--in vessels that are unaffected by microwave energy. Thus, microwave radiation tends to heat chemical samples very quickly. As a result, certain types of reactions that previously took hours can be carried out in minutes using microwave devices.
As a second advantage, because microwaves heat the reagents rather than their ambient surroundings, the effect of the microwaves is direct rather than indirect.
These same characteristics, however, can cause disadvantages in certain procedures. This is particularly true when combined with the typical techniques for generating the microwave themselves. Those familiar with microwave assisted chemistry devices will recognize that the typical microwave source is a half wave rectified power supply that operates at 50 or 60 cycles per second (hertz). In the U.S. 60 cycles is most common, while in most of the rest of the world, 50 cycles is common. Such devices, when operating at full power, provide that power in pulses. As would be expected, however, full power from a source having a certain power rating (e.g., 300 watts), may be satisfactory for a few chemical reactions, but is often too much power for other reactions, particularly those using small samples. Accordingly, some technique must be used to moderate the power that can be applied to particular chemical reactions.
In one technique, a "linear" power supply can be incorporated; i.e., one for which the source's power level is widely adjustable. Such systems, however, require circuitry that bleeds off the excess energy as heat. The systems are large and cumbersome, making them generally impractical for the bench top chemical applications that are used with microwave assisted chemistry. Some present commercial microwave assisted chemistry devices use one to three capacitors to moderate the amplitude of the rectified waves at one, two, or three levels. Nevertheless, incorporating enough capacitors to give a full range of wave amplitude would be highly impractical given the present technology and economics.
Accordingly, a more typical technique for moderating power is to use a single amplitude of power, while moderating the amount of time during which the power is applied in order to obtain a desired average (rather than continuous) power level. For example, if 100 watts of average power are desired from a 300-watt power supply, the power supply is pulsed for a fraction of time that corresponds to the fraction of power desired. Thus, obtaining 100 watts of average power from a 300-watt power supply requires pulsing the power supply "on" for one third of its normal cycle and then "off" for two thirds of its cycle. Because, for example, most power supplies for bench top microwave assisted chemistry devices use alternating current and provide microwaves at a 60 hertz frequency, the shortest time period during which a pulse of power can be on or off is 1/60 of a second (0.0167 seconds). Thus to obtain 100 watts average power from a 300 watt supply, the typical technique applies 300 watts of power for one pulse period (0.0167 seconds), and then turns the power off for the next two pulse periods (a total of 0.0333 seconds). As a result, the average power over the three pulse periods is 100 watts.
It will be immediately recognized, however, that although the average power was 100 watts, in reality 300 watts were applied on an on-and-off basis for repeated short time periods. This application of full power, even for short time periods, has particular disadvantages. These become even more exaggerated when lower average powers are required. For example, when a 5-watt average power is desired or required, it represents 1/60 of the rated 300-watt power supply. Accordingly, the power supply would be pulsed on for one cycle and then off for 59 cycles to produce an average power of 5 watts; e.g., 300 watts divided by 60 time periods. It will thus be recognized that no time period existed in which 5 watts were applied, but instead 300 watts were applied for a very short period of time.
Applying high power for short time periods to create an average power offers significant disadvantages in certain circumstances. First, as noted above, microwaves apply energy directly to the sample and thus the results are often immediate rather that gradual or gentle. Thus, in a reaction where a small amount of power is required, the application of high power, even for a very short time period, can push the reaction past the desired point. In particular, the high power can supply enough energy to drive a reaction past the activation energy for an undesired associated reaction. As one example, the Kjeldahl technique is often used to determine the amount of nitrogen in a sample by converting the nitrogen to ammonia (in several steps) and then measuring the amount of ammonia. If too much microwave power is applied, however, the nitrogen can be overoxidized to an oxidation state that doesn't properly convert to ammonia, and the measured amount of ammonia does not properly reflect the amount of nitrogen that was originally present in the sample.
Additionally, the application of full power pulses to obtain average power can cause localized overheating. Although this does not represent a problem in some reactions, it can raise significant problems in others. In turn, other parameters, such as pressure, can be driven beyond the desired parameters for a particular reaction.
As another disadvantage, microwaves aren't always uniformly absorbed by liquid reagents. The non-uniformity of their absorption can accordingly give a discontinuous or non-representative reading to the control vessels frequently used in microwave systems.
Furthermore, the use of pulses in the conventional sense can preclude the use of microwave chemistry with certain sensitive or more sophisticated reactions that could otherwise be carried out with microwave assistance.
As another disadvantage, microwave assisted chemistry is often carried out simultaneously on a plurality of vessels in a single cavity. The vessels typically rotate on a turntable in an attempt to get the microwaves to distribute evenly among the reagents. The turntable, however, does not guarantee uniform distribution of microwave energy throughout a cavity. Furthermore, the presence of the samples, as well as the changes they undergo during microwave assisted chemistry, also change the energy distribution in the cavity.
Those familiar with microwave assisted chemistry techniques recognize that the microwaves are typically generated by a magnetron, and then are carried through a wave guide to the point at which they enter the cavity (sometimes referred as the "launcher"). In typical systems, the turntable rotates at a relatively moderate speed, for example about every 7 seconds. As noted above, if a relatively low average power (e.g., 5 or 10 watts) is desired from a typical power supply of about 300 watts, the microwaves are pulsed on for only about 16 milliseconds out of every second. When combined with the uncertain wave distribution among the vessels and samples in the cavity, the result can be a surprisingly wide power variation from sample to sample, with an associated wide variation in the progress and results of the intended chemical reactions. These uneven results are often further exacerbated because in typical systems only one vessel is used to give feedback (because of the complexity of measuring feedback for every vessel). Thus, the power is uniformly pulsed for all of the vessels based solely upon indications from the control vessel.
The net effect is that the pulsed power tends to be "off" much more than has been previously recognized. In exaggerated terms, the control vessel is being "smashed" with large amount of power for short periods of time, which is then turned off for relatively long periods of time. As a result, the reactions in the remaining unmonitored vessels may be experiencing quite different conditions, or reaching quite different stages of progress or completion, than the sample vessel.
As an additional problem, the power needed to heat a sample to a particular temperature is generally much greater than the energy required to maintain it at such temperature for a desired period of time while the reaction continues.
Accordingly, the need exists for a technique and apparatus for carrying out microwave assisted chemical reactions that avoids the undesired effects of applying large amounts of power for short periods of time in an attempt to obtain an average power, and an associated need likewise exists for a technique for concurrently heating multiple vessels that avoids the pitfalls associated with attempting to control the application of larger pulsed microwave power to multiple vessels using a single control vessel on intermittent basis.